The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to real-time MR image processing, including a method and apparatus to perform MR image subtraction on-the-fly.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In contrast enhanced MR angiography (MRA), MR images are obtained of an artery or other blood carrying vessel in the peripheral vasculature of a patient. Data is usually acquired after an initial test bolus of contrast agent is injected into the patient and is timed as it travels along the vessel or other conduit from one station to the next. After the bolus travel time is known, an exam bolus is injected and MR data is acquired at each scan station where the bolus is located.
Phase contrast MRA is another practical and clinically applicable technique for imaging blood flow. MRA makes use of flow encoding gradient pulses which impart a velocity-dependent phase shift to the transverse magnetization of moving spins while leaving stationary spins unaffected. Each phase contrast acquisition generates two images: a magnitude image that is proportional to the proton density of the object and may also be T1-weighted, and an image representing the phase of the object. The phase image produced has information only from the moving spins and the surrounding stationary tissue is suppressed. Images representing both the average flow over the entire cardiac cycle and at a series of individual points in the cycle have been generated using this technique. The phase contrast MR method produces phase images with intensities that represent the magnitude of the flow velocity and also the direction of flow. Therefore, such images may be used for both qualitative observation of blood flow and quantitative measurement. The practical application of phase contrast MR angiography and venography to the quantitative determination of flow velocity is therefore evident.
The most common approach to MRA involves collecting a mask image first, then collecting a series of images subsequent, and then subtracting the mask image from each of the images in the series after image reconstruction. For near real-time applications, all of the images are transferred to a separate workstation where a subtraction is performed, and the subtracted images are transferred back to the operator""s console for display.
In these xe2x80x9cnear real-timexe2x80x9d subtraction angiography techniques, which are typically done on a second workstation, a reference or baseline image is first collected, which is then transferred to the second workstation and stored, either as a positive or a negative image. Data is then collected during subsequent passes and each is individually transferred to the second workstation. Thereafter, after all the data is collected, a subtraction of the images is performed offline. Once the data is subtracted, the image can be displayed either on the second workstation or sent back to the operator""s console, as desired.
It would therefore be desirable to have a method and apparatus for real-time data subtraction during acquisition with minimal computation time such that images can be reconstructed on-the-fly and displayed on an operator""s console.
The present invention relates to a system and method of real-time image processing including MR image subtraction and reconstruction that overcomes the aforementioned problems.
The invention includes an MR digital processing technique wherein a mask image is copied forward multiple times in acquisition memory. Subsequently acquired data is automatically subtracted on-the-fly, thereby yielding images without the typical prior art penalties involved in image reconstruction and image post-processing time.
In accordance with one aspect of the invention, a method of real-time MR image processing includes acquiring an MR mask image comprised of k-space line data and copying the k-space line data of the MR mask image into a number of different memory locations, the number of which corresponds to a preselected number of MR image acquisitions, as set by an MR operator. The method next includes acquiring the preselected number of MR images and accumulating the k-space line data of each acquired MR image in a corresponding memory location with the k-space line data of the MR mask image. By setting a polarity of the stored data, or the newly acquired data, the present invention provides a method for real-time subtraction during acquisition without post-processing or the use of a separate processing station. Computation time is minimal due to the subtraction occurring in-place as each line of k-space is acquired, and therefore, the subtracted images are reconstructed on-the-fly. Either positive or negative reverse contrast images, are easily obtainable by simply setting a flag to set the polarity of the mask data that is copied forward into memory.
In accordance with another aspect of the invention, an MRI apparatus to process MR images in real-time is disclosed in which a magnetic resonance imaging system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field has an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images, and a computer programmed to acquire the MR mask image data and copy the acquired MR mask image data into a number of different memory locations. The computer is then programmed to acquire a predetermined number of MR image data sets and then accumulate the acquired MR image data sets in the same memory locations as the acquired MR mask image data.
In accordance with yet another aspect of the invention, a computer memory medium is disclosed having thereon a computer program for use with an MRI apparatus which, when executed, causes a computer to acquire MR mask image data and copy that acquired MR mask image data into a number of different memory locations. The computer is also programmed to acquire MR image data for a number of image acquisitions as set by an operator and then accumulate the acquired MR image data into the same memory locations as the acquired MR mask image data previously saved.
The present invention is particularly useful in MR angiography where digital subtraction of MR images has heretofore been time consuming and memory intensive. Use of the present invention in MR angiography provides a fast digital subtraction of a mask image and subsequently acquired data automatically and on-the-fly, thereby yielding fluoroscopic images without the typical penalties in image reconstruction or image post-processing time.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.